The present invention relates to an apparatus and method for controlling a semiconductor laser module.
The explosive popularization of the Internet has been accompanied by a remarkable increase in transmission capacity needed for backbone systems. The importance of high-density wavelength multiplexing optical-fiber transmission in terms of raising transmission capacity is growing and semiconductor lasers used in such optical-fiber transmission require both stable optical power and wavelength.
Conventionally, semiconductor lasers used in such fields as optical communications and optical measurement emit light in two directions. The light in one direction is sensed by a photodiode and the driving current of the semiconductor laser is controlled so as to render constant the amount of current through the photodiode, thereby stabilizing the light emitted in the other direction. This makes it possible to exercise control to increase the driving current of the semiconductor laser and hold the strength of the output light constant even if the semiconductor laser deteriorates with time.
Owing to the passage of driving current through the semiconductor laser, the temperature thereof rises and the refractive index of the semiconductor laser element increases, thereby causing the lasing wavelength to shift toward the long-wavelength side. A method often employed to solve this problem includes disposing a thermistor on the carrier of the semiconductor laser to sense temperature and cooling the semiconductor laser by an electronic cooling device using a Peltier element.
An apparatus of this kind for controlling a semiconductor laser module will be described with reference to FIG. 3.
FIG. 3 is a block diagram illustrating an example of the prior art in which an automatic power control (APC) circuit 4 and an automatic temperature control (ATC) circuit 5 are connected to a semiconductor laser module.
The APC circuit 4, which is for rendering constant the optical output power of a semiconductor laser 1, will be described first.
The semiconductor laser 1 emits laser beams from both ends in directions extending to the right and left in FIG. 3. Backward-emitted light 8 on the right side is used to exercise control in such a manner that the output power of forward-emitted light 7 on the left side is rendered constant. The backward-emitted light 8 is received by a photodiode 2 and photoelectrically converted to a monitor current 10, which is then input to the APC circuit 4. The latter controls a laser driving current 9, which is output to the semiconductor laser 1, so as to render the value of the monitor current 10 constant, thereby rendering constant the forward-emitted light 7. In other words, the laser driving current 9 is control led in such a manner that the intensity of the laser light is rendered constant regardless of the wavelength of the laser emission.
The ATC circuit 5 for controlling the temperature of the semiconductor laser 1 will be described next.
A thermistor 6 is placed on the carrier in close proximity to the semiconductor laser 1 in order to sense the temperature of the semiconductor laser 1. The ATC circuit 5 senses the resistance value of the thermistor 6 and passes a driving current 12 into an electronic cooling device 3 in such a manner that the resistance value attains a reference resistance value, thereby holding the temperature of the semiconductor laser 1 constant.
More specifically, the ATC circuit 5 passes the driving current 12 through the cooling device 3 in a direction that cools the thermistor 6 if the temperature sensed by the thermistor 6 is higher than a set temperature, and passes a current in a direction that heats the thermistor 6 if the temperature sensed thereby is lower than the set temperature. The ATC circuit 5 controls temperature in such a manner that the value of the passed current increases if the difference between the sensed temperature of the thermistor 6 and the set temperature is large and decreases if the temperature difference is small. Thus, the ATC circuit 5 exercises control in such a manner that the temperature of semiconductor laser 1 remains constant independently of the optical output power of the semiconductor laser 1, i.e., independently of the magnitude of the laser driving current 9.
Accordingly, the APC circuit 4 controls the laser driving current 9 in such a manner that the monitor current 10 of the photodiode 2 remains constant even if a change in the temperature of the semiconductor laser 1 is accompanied by a change in the wavelength characteristic of the output light. The APC circuit 4 thus operates so as to hold the intensity of the output light constant. On the other hand, the ATC circuit 5 causes the cooling device 3 to operate independently of the APC circuit 4 to hold the temperature of the semiconductor laser 1 constant, regardless of the magnitude of the laser driving current 9, if the temperature of the semiconductor laser unit varies.
However, the following problems have been encountered in the course of investigation toward the present invention. Namely, the conventional control apparatus of this construction for controlling a semiconductor laser module has a certain drawback relating to a shift in wavelength. Specifically, when the semiconductor laser 1 begins to age, the lasing threshold-value current increases and there is a corresponding increase in the laser driving current 9 for obtaining the desired optical output power, the actual rise in the temperature of the semiconductor laser 1 and the reading (value) of the temperature rise of thermistor 6 are no longer the same owing to the existence of a thermal resistance between the semiconductor laser 1 and a carrier part on which the thermistor 6 is placed. Even if the temperature of the thermistor 6 is held constant, therefore, the semiconductor laser 1 assumes a temperature higher by an amount commensurate with the value of the thermal resistance. As a result of this higher temperature, the lasing wavelength of the semiconductor laser 1 is shifted toward the side of longer to wavelengths owing to a change in index of refraction.
This problem associated with the rise in the temperature of the semiconductor laser 1 that accompanies the change in the laser driving current 9 is particularly significant in high-density wavelength-multiplexing optical communication. The reason is that the change in wavelength accompanying the temperature rise of the semiconductor laser 1 causes crosstalk and a decline in reception sensitivity, thereby degrading the transmission characteristics.
Accordingly, an object of the present invention is to provide an apparatus and method for controlling a semiconductor laser module wherein even if laser driving current increases owing to aging of the semiconductor laser, temperature control can be performed accurately so as to render constant the actual temperature of the semiconductor laser and prevent a change in lasing wavelength that accompanies a change in temperature.
According to a first aspect of the present invention, the foregoing object is attained by providing an apparatus for controlling a (e.g., semiconductor) laser module, comprising: a laser (particularly semiconductor laser), an optical sensor for sensing optical intensity of the laser; optical-power stabilizer for controlling driving current of the semiconductor laser in accordance with an output from the optical sensor; a temperature sensor disposed in the proximity of the laser for sensing the temperature thereof; temperature controller for driving an electronic cooling device, which cools the semiconductor laser, in accordance with an output from the temperature sensor; and a module of predicting actual temperature of the laser from information indicative of the driving current of the laser and temperature information output from the temperature sensor.
According to a second aspect of the present invention, the foregoing object is attained by providing an apparatus for controlling a (e.g., semiconductor) laser module, comprising: a laser (particularly semiconductor laser), an optical sensor for sensing optical intensity of the laser; optical-power stabilizer for controlling driving current of the semiconductor laser in accordance with an output from the optical sensor; a temperature sensor placed in the proximity of the laser for sensing the temperature thereof; temperature controller for driving an electronic cooling device, which cools the laser, in accordance with an output from the temperature sensor; and a current-quantity sensor, which is connected to the optical-power stabilizer and temperature controller, for sensing the driving current of the laser output from the optical-power stabilizer; wherein data obtained by actual measurement of actual temperature of the laser in relation to the driving current of the laser is stored in the temperature controller in advance, and the driving current of the cooling device is set upon comparing the data obtained by actual measurement, laser driving current information output from the current-quantity sensor, and temperature information output from the temperature sensor.
According to a third aspect of the present invention, the foregoing object is attained by providing a method of controlling a (e.g., semiconductor) laser module wherein driving current of a laser (particularly semiconductor laser), is controlled by optical-power stabilizer on the basis of an output from an optical sensor which senses optical intensity of the laser, and an electronic cooling device for cooling the laser is driven by temperature controller on the basis of on output of a temperature sensor placed in the proximity of the laser. The method comprises: previously storing, in the temperature controller, data obtained by actual measurement of actual temperature of the laser in relation to the driving current of the laser when the electronic cooling device is driven; comparing the data obtained by actual measurement, laser driving current information output from the optical-power stabilizer and temperature information output from the temperature sensor; and driving the electronic cooling device of the basis of the comparison in such a manner that the actual temperature of the laser is rendered constant.